fix(bienvenida,grabaciones): headline size, CTA radius, storage caption

Screen 14 / audit 14.3+14.7: welcome headline now 34px/ls-1.2 (was
headlineMedium's 28/ls-1.0), CTA restyled to a radius-18 rounded
rectangle instead of Material 3's default StadiumBorder (t4:696,715).

Screen 12 / audit 12.2+12.3: the recordings storage card now shows the
bold "X of Y used" headline ABOVE a 6px/radius-3 bar, followed by a
real folder-path + purge-policy caption below it -- was the bar first
with the "used" string as its only (small, generic) caption (t4:613).
New ARB key recordingsLibraryStorageFolderCaption, translated to all
13 locales.

Item 12.5 (folder/max-size settings living behind the header action
instead of inline on this screen) is a deliberate structural split
from WU15b; the audit itself scopes it out of this pass.

test/pantallas/pantalla_bienvenida_test.dart

@@ -133,6 +133,42 @@ void main() {
     },
   );
 
+  testWidgets('visual fidelity (audit 14.3): the headline is 34px/w800/ls-1.2 '
+      '(t4 line 696), not headlineMedium\'s 28', (tester) async {
+    final navegacion = EstadoNavegacionRaiz();
+    addTearDown(navegacion.dispose);
+
+    await tester.pumpWidget(_app(navegacion: navegacion));
+    await tester.pumpAndSettle();
+
+    final headline = tester.widget<Text>(find.text('Tu mundo, en directo'));
+    expect(headline.style?.fontSize, 34);
+    expect(headline.style?.fontWeight, FontWeight.w800);
+    expect(headline.style?.letterSpacing, -1.2);
+  });
+
+  testWidgets('visual fidelity (audit 14.7): the CTA is a radius-18 rounded '
+      'rectangle (t4 line 715), not the default fully-round StadiumBorder', (
+    tester,
+  ) async {
+    final navegacion = EstadoNavegacionRaiz();
+    addTearDown(navegacion.dispose);
+
+    await tester.pumpWidget(_app(navegacion: navegacion));
+    await tester.pumpAndSettle();
+
+    final boton = tester.widget<FilledButton>(find.byType(FilledButton));
+    final forma = boton.style?.shape?.resolve(<WidgetState>{});
+    expect(
+      forma,
+      isA<RoundedRectangleBorder>().having(
+        (s) => (s.borderRadius as BorderRadius).topLeft,
+        'topLeft radius',
+        const Radius.circular(18),
+      ),
+    );
+  });
+
   testWidgets('rendered content contains no monetization strings', (
     tester,
   ) async {